1. Field of the Invention
This invention relates to color cathode ray picture tubes, and is addressed specifically to an improved process for the manufacture of tubes having a tensed foil shadow mask. Color tubes of various types that have a tension foil mask can be manufactured by the process, including those used in home entertainment television receivers. The process according to the invention is particularly valuable in the manufacture of medium-resolution, high-resolution, and ultra-high resolution tubes intended for color monitors.
The use of the foil-type flat tension mask and flat faceplate provides significant benefits in comparison to the conventional domed shadow mask and correlatively curved faceplate. Chief among these is a greater power-handling ability which makes possible as much as a three-fold in-crease in brightness. The conventional curved shadow mask, which is not under tension, tends to "dome" in picture areas of high brightness where the intensity of the electron beam bombardment is greatest. Color impurities result as the mask moves closer to the faceplate and as the beam-passing apertures move out of registration with their associated phosphor elements on the faceplate. When heated, the tensed mask distorts in a manner quite different from the conventional mask. If the entire mask is heated uniformly, there is no doming and no distortion until tension is completely lost; just before that point, wrinkling may occur in the corners. If only portions of the mask are heated, those portions expand, and the unheated portions contract, resulting in displacements within the plane of the mask; i.e., the mask remains flat.
The tensed foil shadow mask is a part of the cathode ray tube faceplate assembly, and is located in close adjacency to the faceplate. The faceplate assembly comprises the faceplate with its screen, which consists of deposits of light-emitting phosphors, a shadow mask, and support means for the mask. As used herein, the term "shadow mask" means an apertured metallic foil which may, by way of example, be about 0.001 inch or less in thickness. The mask must be supported under high tension a predetermined distance from the inner surface of the cathode ray tube faceplate; this distance is known as the "Q-distance." As is well known in the art, the shadow mask acts as a color-selection electrode, or "parallax barrier," that ensures that each of the three electron beams lands only on its assigned phosphor elements.
The conventional process of depositing patterns of color phosphor elements on the screening surface of a color picture tube faceplate utilizes the well-known photoscreening process. A shadow mask, which in effect functions as a perforated optical stencil, is used in conjunction with a light source to expose in successive steps, at least three light-sensitive photoresist patterns on the screening surface. The shadow mask is typically "mated" to each faceplate; that is, the same mask is used in the production of a specific tube throughout the production process, and is permanently installed in the tube in final assembly. Typically, four engagements and four disengagements of the mask, as well as six exposures, are required in the standard photoscreening process. In certain of the processes, a "master" may be used for exposing the photo-resist patterns in lieu of a shadow mask permanently mated to the faceplate and its screen.
2. Prior Art
Kautz et al in U.S. Pat. No. 2,916,644 discloses a face panel assembly adapted to cooperate with a photoexposure device used in forming the screen. In one aspect of Kautz et al, spaced tabs are welded on or otherwise attached to the flange, or apron, of a curved face panel, and may extend either inwardly or outwardly. The tabs have apertures for cooperating with referencing means on the exposure table. An aligning device comprises a number of spheres each positioned in a groove. The means of aligning according to Kautz et al also provides for aligning a face panel assembly, a shield, and a funnel with one another in the finished tube.
U.S. Pat. No. 3,894,321 to Moore, of common ownership herewith, is directed to a method of producing a color cathode ray tube having a a funnel section, a foil shadow mask attached to a support, and a faceplate for receiving deposits of light-emitting phosphors. The foil is stretched across a supporting frame, and the faceplate and frame are aligned on an exposure table for photoscreening of the faceplate. The exposure table is tilted and the components are positioned by contact with alignment posts, and held there by gravity during the photoscreening process.
U.S. Pat. No. 4,100,451 to Palac, which is assigned to the assignee of the present invention, describes a system for suspending a domed non-self-rigid shadow mask a predetermined distance from a faceplate. Four suspension means provide for coupling and indexing the mask directly to corner portions of the faceplate. In one embodiment, the indexing means comprise legs having rounded portions which engage indexing cavities in the faceplate which may be in the form of V-grooves or slots. Another approach utilizes V-blocks at the four corners of the faceplate, each of which has a clamp attached to the mask. Each clamp has a foot for mating with a V-block. The suspension and indexing means provide for the permanent mounting of the shadow mask in relation to the faceplate, as well as for the temporary mounting of the mask during the production screening process.
External referencing means for a tube having a tensed foil mask is described and claimed in referent U.S. Pat. No. 4,595,857, of common ownership herewith. The referencing means provide for the precise faceplate-mask registration. The faceplate is equipped with three externally mounted, outwardly directed, breakaway pins. Indexing means cooperating with each of the pins comprises a breakaway tab affixed to a frame member which supports the shadow mask. Each tab has a depending finger which is provided with a bifurcation at its distal end. To effect registration, the faceplate is located so that the finger bifurcations are poised over the assigned pins. When the assembly is mated, a six-point contact is established between the three pins and their cooperating bifurcations. This registration is repeatable as often as is required to accomplish the screening process, as well as to effect a final registration between the electrode assembly and the faceplate during frit sealing. After frit sealing, the pins and tabs are removable; that is, they can be broken away.
In U.S. Pat. No. 4,713,034 to Lee et al., of common ownership herewith, there are disclosed components-in process and assemblies-in-process for use in the manufacture of a high-resolution color cathode ray tube having a tensed foil shadow mask. A first component-in-process comprises a faceplate with a target area for receiving at least one pattern of phosphor deposits. The faceplate has attached at preselected, widely spaced locations on the sides thereof a plurality of discrete, detachable first indexing elements. A second component-in-process comprises a shadow mask support assembly having a frame ultimately constituting a part of the tube envelope; the frame supports the shadow mask in precise adjacency to the target area. The frame has attached on the sides thereof a like plurality of discrete, detachable second indexing elements which are in facing adjacency to the first indexing elements on the faceplate when the faceplate and frame are mated. Temporary attachment of the indexing elements is by means of a thermally degradab1e cement. The faceplate and frame can be inter-registered in precise relationship by the temporary use of the first and second indexing elements in the process of screening the pattern of phosphor deposits on the target area and later in the final assembly of the tube.